Question: Which of these numbers is prime? ${15,\ 39,\ 44,\ 61,\ 63}$
Explanation: A prime number has exactly two factors: 1 and itself. The factors of 15 are 1, 3, 5, and 15. The factors of 39 are 1, 3, 13, and 39. The factors of 44 are 1, 2, 4, 11, 22, and 44. The factors of 61 are 1 and 61. The factors of 63 are 1, 3, 7, 9, 21, and 63. Thus, 61 is a prime number.